This invention relates generally to low softening point, low thermal expansion coefficient seal glasses, and more particularly to a composition and process for preparing a copper oxide containing glass useful in forming gas tight glass seals and the glass compositions prepared thereby.
Low softening point, low thermal expansion coefficient, copper containing glass compositions are useful in sealing various surfaces such as ceramics, metal, and glasses and for joining such surfaces together. Such copper glasses are used, for example, to seal together the glass substrates used to form gas display panels. The glasses must have a softening point which will permit the seal to be made at a temperature below the point at which thermal damage will occur to the substrate and which have thermal expansion properties so that it will be compatible with the soda-lime-silica glass conventionally used as substrates in the manufacture of the gas panels.
Low softening temperature copper-oxide containing seal glass compositions and methods for their preparation are described, for example, in U.S. Pat. Nos. 3,873,330, 3,904,426, and in copending application, Ser. No. 572,036, which was filed on Apr. 28, 1975. The glasses are formed from a mixture of oxides including cupric oxide (CuO). One problem associated with making such glasses is the conversion of cupric oxide, CuO, to the cuprous form, Cu.sub.2 O, during the glass forming process. This is believed to be the cause of the formation of non-homogeneous glasses which contain bubbles, seeds, lumps, and Cu.sub.2 O precipitates. These bubbles, seeds, lumps, and precipitates can cause either contamination of the gas panels and/or porous seals to be formed during the sealing process. The aforementioned patents and application address this problem from the standpoint of reducing or avoiding such difficulties by keeping the conversion of the copper oxide from the cupric to the cuprous form to a minimum. The presence of cuprous oxide, in other words, has heretofore been regarded as undesirable.
It has now been found that the problems heretofore associated with the presence of Cu.sub.2 O in the sealing glass do not occur if the cupric oxide in the original mixture of oxides is replaced ab initio by cuprous oxide. It also has the additional advantage of producing a sealing glass compositions which have lower softening points than comparable compositions having the copper in the form of cupric oxide.